The present invention relates to an apparatus for coating a substrate, in particular a printed circuit board, with a device for applying a coating material (material applying device), in particular a protective coating, and a device for supplying a gaseous medium (gas supplying device), the material applying device having an inner tubular element. The invention also relates to a method for coating a substrate with a coating material, preferably a printed circuit board with a protective coating.
Apparatuses and methods of the aforementioned type are generally known and are used for example for providing printed circuit boards with a protective coating. A major problem when coating printed circuit boards with a protective coating is in particular that of applying the protective coating accurately in just the right places. Small deviations or splashes of protective coating can very quickly lead to soiling of contacts, which later has the effect of malfunctions of the circuit.
Generally, so-called spraying methods are often used, operating with differently shaped spray nozzles to atomize a coating as it leaves a nozzle. Air-assisted atomizing of the coating produces a coating film that is complete in the center, but depleted toward the outside, forming a spray mist and splashes. This method of spraying is particular suitable for manual spraying by means of paint spray guns, because of the seamless transitions. Broad overlapping of the spray coats is a prerequisite for a uniform spray pattern, therefore no clearly delimiting coating edge can be formed.
A coating pattern with exact delimitation of the coating area and without a spray mist can be obtained with so-called airless spraying systems. Because these require exact, accurately repeatable spraying, the spray valves used are guided by robot systems. The overlapping of the spray coats should in this case be as small as possible, to avoid accumulations of material. In the prior art, different spray valves and constructions are known.
A known method for applying a protective coating is known by the name “Select-Coat? method” and is based on a slot nozzle which has a round hole on both sides. The protective coating is forced through this nozzle. Maintaining the coating film is made possible by the two streams of coating from the round holes. Substrates with relatively high obstacles, as represented for example by a printed circuit board, can only be coated unsatisfactorily, because, depending on the speed and height of the component, spray shadows form behind each obstacle passed over. Splashes also occur under these conditions. The viscosity of the coating must not exceed 200 mPas. Otherwise, the formation of splashes increases.
For higher-viscosity coating materials there exists a method which is known as the “Swirl-Coat method” and is based on a spray jet moved in a circular or elliptical manner. This method is well suited in the case of printed circuit boards for areas fitted with low components, whereas greatly varying layer thicknesses are produced on areas fitted with higher components. The application width of the sprayed area also changes with the unavoidably necessary changing of the distance of the nozzle from the substrate. This method is not suitable for the application of low-viscosity materials and coatings to printed circuit boards, because excessive differences in the height of the coating are produced. The spraying head cannot enter between high components because of its size.
DE 33 29 880 A1 discloses a spray valve which has a material nozzle, which distributes material at the nozzle outlet, and air nozzles, which direct the material stream of finely distributed droplets that is produced. Round or elliptical coating areas corresponding to the directing of the air stream are obtained and form coating paths. The spraying head cannot enter between high components because of its size. With the resultant increasing distance, the spray jet widens and produces splashes.
U.S. Pat. No. 6,170,760 B1 discloses an apparatus in which the air is guided around the coating material in the material nozzle in the form of an envelope. The coating is atomized as it leaves the spray valve. The nozzle may be made thin but not of any desired length, because otherwise the pulsation effect affecting this method intensifies. An excessively thin nozzle (<0.6 mm) also intensifies the pulsation and produces splashes. The pulsation occurs because the coating material tends toward the cone shape which it adopts in the air stream according to the speed, nozzle diameter and viscosity of the coating material. The air expanding as it leaves the nozzle produces splashes if the nozzle is too thin. For this reason, a narrow coating area cannot be produced, but only paths about 20 mm wide. Only low-viscosity coatings can be handled. With materials of higher viscosity, the pulsation increases, with the consequence that splashes are produced, because both air and material accumulations form in the spray nozzle, leading to irregular material delivery.
U.S. Pat. No. 6,132,809 A discloses a rotatable dual head, which is equipped with a dispensing head and a spraying head.